1. Field Of The Invention
This invention relates to the sealing of alumina arc tubes for arc discharge lamps and in particular to high pressure sodium (HPS) vapor lamps.
2. Description Of The Prior Art
Within the past few years, high pressure sodium vapor lamps have become commercially useful, especially for outdoor lightining applications, because of their high efficiency, generally in excess of 100 lumens per watt. The sodium operating vapor pressure in such lamps is of the order of about 50 to 100 Torr.
Such lamps are called high pressure in order to distinguish them from low pressure sodium vapor lamps in which the sodium operating vapor pressure is in the order of a few microns. Low pressure sodium lamps have been in use for about 30 years but, although efficient, they produce an unattractive monochromatic yellow light. The color of light from HPS lamps is considerably improved over that from low pressure sodium lamps.
HPS lamps generally comprise an alumina ceramic arc tube containing a fill including sodium, mercury and an inert gas. Disposed at the ends of the arc tubes are metal electrodes and the ends of the arc tube are sealed with ceramic to metal seals.
A common method of manufacture of the arc tube involves the use of an alumina washer. The washer is disposed on the end of the arc tube with a glass seal ring therebetween. A niobium tube, containing an electrode on its inner end, is then inserted through a hole in the alumina washer with part of the niobium tube protruding externally. A second glass sealing ring is disposed around the niobium tube and rests on the alumina washer. The assembly is then heated to the melting point of the glass sealing rings to form the ceramic to metal seal. The first ring seals the alumina washer to the arc tube. The second ring seals the niobium tube to the alumina washer.
A problem resulting from this method of manufacture is the deposition of an undesirable dark material at the ends of the arc tube. This deposit is undesirable since it reduces the light transmission of the arc tube. The deposit occurs as a result of a reaction between the glass sealing material and the niobium tube.